The life-cycle of the human immunodeficiency virus (both HIV-1 and HIV-2) is well known. HIV primarily infects cells of the human immune system, such as helper T cells (specifically CD4+ T cells), macrophages and dendritic cells. Entry to cells of the immune system is mediated through interaction of the virion envelope glycoproteins (gp120 and gp41) with the receptor CD4 on the target cells. In addition, viral entry is modulated through at least two co-receptors known as CXCR4 and CCR5, which are members of the chemokine receptor family of proteins, and have been shown to function with CD4 as coreceptors for HIV-1 isolates that are tropic for T-cell lines or macrophages, respectively (Feng et al., 1996, Science 272:872-876; Alkhatib et al., 1996, Science 272:1955-1958; Deng et al., 1996, Nature 381:661-666; Dragic et al., 1996, Nature 381:667-673). Other molecules in this family including CCR3 and CCR2b, also appear to function as cofactors for some HIV-1 isolates (Doranz et al., 1996, Cell 85:1149-1158; Berson et al., 1997, J. Virol. 71: 1692-1696; Choe et al., 1996, Cell 85:1135-1148).
Current anti-HIV therapy includes the use of compounds which inhibit various aspects of the HIV life-cycle, including entry, fusion and replication in a target cell. While these therapies, particularly when used in combination with one another, are effective, they are frequently short-lived in that the viral strains rapidly develop resistance to one or more of the compounds used—a widespread and major problem in the current approaches in treating HIV infections.
Antibodies represent yet another promising approach in the treatment of HIV infections. Human monoclonal antibodies (mAbs) currently represent an important and growing technology in the development of inhibitors, vaccines, diagnostic and research tools. In fact, 22 mAbs have been approved by the US Food and Drug Administration against various disease in the past several decades for various disease indications, including rheumatoid arthritis (Centacor's REMICADE and Abbott Laboratories' HUMIRA), non-Hodgkin's lymphoma (Genentech's RITUXAN and IDEC's ZEVALIN) and respiratory syncytial virus infection (Medimmune's SYNAGIS). Many other antibody drug candidates are in the late stages of clinical trials and, as such, antibodies are now well established as both highly potent and well tolerated therapeutics.
However, no mAbs have yet been approved for clinical use against HIV-1. A fundamental problem in the development of HIV-1-neutralizing antibodies is the virus's innate ability to escape human immune surveillance during the long chronic infection. Several known mAbs, however, have been shown to exhibit potent and broad HIV-1 neutralizing activity in vitro, and can prevent HIV-1 infection in animal models (reviewed in Burton, 2002, Ferrantelli et al., 2002, and Veazey et al., 2003). A recent clinical trial suggested that two of these broadly HIV-1 neutralizing human mAbs, 2F5 and 2G12, lack side effects in humans (Armbruster et al., 2002; Stiegler et al., 2002). However, the potency of 2F5 and 2G12 used in combination in this clinical trial was significantly lower than currently available treatments and relapses occurred (Stiegler et al., 2002). Further increase in the potency of anti-HIV antibodies and/or new, more effective anti-HIV antibodies would be a significant advancement in the art.
Another fundamental problem in the development of effective therapeutic antibodies against HIV is the problem of epitope accessibility. It has been reported that some epitopes are sterically inaccessible to full size antibodies. For example, a study relating to HIV CD4-inducible (CD4i) epitopes by one of the present inventors has suggested that the size of the CD4i-specific neutralizing antibodies inversely correlates with neutralization efficiency and that perhaps antibody fragments might be more effective than whole antibodies in neutralizing the virus at such epitopes. See Labrijn et al., J. Virol., 2003. The study suggests that HIV's ability to evade the host's immune system may be linked in part to its having found a way to sterically hinder full-sized antibodies from accessing the CD4i epitopes. This study was limited, however, to exploring the effectiveness of scFv and Fab antibody fragments.
In the late 1980s, domain antibodies were identified as the smallest known antigen-binding fragments (Holt et al., 2003). Structurally, domain antibodies comprise the single chain variable heavy (VH) or variable light (VL) polypeptides, and due to their single-chain nature, range in size of only 11 kDa to 15 kDa. Domain antibodies, however, have a number of acknowledged problems to overcome to be suitable as potential therapeutics. Domain antibodies, particularly those derived from human antibodies, suffer from poor stability and solubility, and have a tendency to aggregate due to exposed regions of hydrophobicity in the absence of the paired VH or VL.
New and effective domain antibodies which would overcome the problems in the art, and an effective means of identifying and obtaining such domain antibodies, would be a valuable advance in the art. Such antibodies could be the basis of new methods and approaches for treating and/or prophylaxis of a variety of infections and conditions, such as HIV or cancer, in particular, infections and conditions which are capable of evading the immune system or therapeutic compounds and antibodies because certain epitope targets are sterically restricted.